The IEC 1000-3-2 regulation demand that higher output power converters (currently over 75 W) must have the front-end active Power Factor Correction (PFC) to meet the stringent requirements for the input current harmonics content. Thus, the front-end boost converter with active PFC feature is the most common solution present in the higher power designs of the AC-to-DC converters. High input line voltage causes that the main switch in the converter is exposed to high voltage stress, during switching transitions, caused by the very high voltage spikes superimposed on the already high rectified input line voltage. This in turn requires that the main MOSFET switch has a much higher voltage rating to sustain safely this overvoltage stress, which not only increases the cost of the component but also degrades efficiency performance of the converter. The MOSFET switch with higher voltage rating has the higher RON resistance and higher conduction losses, which reduces overall efficiency of the converter.
Clearly, elimination of the switching voltage spikes will allow use of the lower voltage rating MOSFETs with lower RON resistance and higher efficiency performance. To achieve this, many different types of snubber circuits were invented, which typically reduce the size of voltage spikes by absorbing and dissipating their energy. These solutions are helpful for the reduced voltage rating of the MOSFET switch, but benefit of reduced conduction losses because of the lower RON resistance is mainly lost because of the additional dissipation losses of the snubber circuit. In fact, the dissipative snubber circuit is easily recognized by the presence of a resistor in the additional snubber network. A typical prior art circuit with dissipative snubber is shown in FIG. 1. Dissipative snubber circuits are now being made obsolete by use of the non-dissipative snubber circuits in which the dissipative resistor is replaced by one or more of the energy storage devices, such as inductors and capacitors, which together with additional switching devices (transistor and diodes) form an energy recovering network to restore the energy which would otherwise be lost in dissipative snubber resistor.
In summary, the snubber circuits can be first classified into two categories:                1. Dissipative snubber circuits        2. Non-dissipative snubber circuits.        
The non-dissipative snubber circuits, in turn, can further be classified into tow categories:                1. Active non-dissipative snubber circuits, which are characterized by the presence of one or more active switching devices, such as MOSFET transistors, which require additional appropriate transistor drive control making these solutions less attractive.        2. Passive non-dissipative snubber circuits, which consists of only passive diode switching devices, thus eliminating the need for additional switching drive signals. The additional diodes are switched in response to the state of the converter rather than the external drive signal as in switching transistors.        
An example of the prior art of the first category is the circuit described in U.S. Pat. No. 5,414,613 issued May 9, 1995 to Keming Chen. This prior art passive snubber circuit has an additional limitation that is limited to the operation of the converter in discontinuous conduction mode, which is not suitable for higher power converter due to its high peak inductor current. The present invention does not have such limitation and is well suited for high power applications, such as 1 kW experimental example described later.
The present invention also belongs to the second category of passive non-dissipative snubber since it does not use additional active switching devices. The prior art passive snubber described in U.S. Pat. No. 5,636,114 by Pradeep Bhagwat et al., uses in addition to an inductor two additional saturable reactors, as opposed to a single saturable reactor of present invention making it more complex and costly to implement.
This invention results in a non-dissipative snubber circuits with very effective reduction of the transitional voltage spikes without power losses which are usually present to certain extent in some of the prior art passive non-dissipative snubber circuits. The switching converters with new non-dissipative snubber with single saturable reactor disclosed herewith eliminates one or more drawbacks of each of the prior art passive snubber circuits and thus results in the higher efficiency, lower EMI noise, and lower voltage rating of the main MOSFET switch. This results in not only the better performance of the converter but also in its lower overall cost and simpler more reliable snubber circuit.